A rule of thumb: The wheel size should be half the diameter of the tire size.
i.e. 16" wheel with a 33" tire, 17" wheel with a 35" tire. 18" wheel with 37" tire, etc....
A larger wheel, and less tire sidewall gives better on-road handling.
A smaller wheel, and more tire sidewall gives better off-road ride (more bounce absorption) and less chance of damaging the wheel.
However, beware of running 15" wheels, you may have to grind the brake caliper corners to fit within the wheels.
Personally, I run 17" wheels on 37" tires.
2008 2-dr Rubicon, auto,
RK 2.5" Max Travel, Rancho 9000XL shocks, 37" Nittos, 5.38's, RR skids, Warn VR8000-S.
RIPP Gen 2 Stg 2 Supercharger.
Political Correctness offends me.